This invention relates generally to control systems for appliances, and more particularly, to calibration and multiplexing of a plurality of sensors used in appliance control systems.
Known household appliances are available in various platforms having different structural features, operational features, and controls. For example, known refrigerator platforms include side-by-side single and double fresh food and freezer compartments, and vertically oriented fresh food and freezer compartments including top mounted freezer compartments, and bottom mounted freezer compartments. Conventionally, a different control system is used in each refrigerator platform. For example, a control system for a side-by-side refrigerator typically controls the fresh food temperature by controlling operation of a mullion damper located between the fresh food compartment and the freezer compartment, a fresh food fan and a variable or multi-speed fan-speed evaporator fan. Top mount refrigerators and bottom mount refrigerators however, are available with and without a mullion damper, the absence or presence of which consequently affects the refrigerator controls. Other major appliances, including dishwashers, washing machines, dryers and ranges, are available in various platforms and employ different control schemes.
Known electronically controlled appliances typically employ a dedicated connection between a controller and a plurality of peripheral devices, including but not limited to sensors to monitor various operating conditions of the appliance. Consequently, it is necessary to calibrate the sensors so that signals from the sensors may be corrected for system measurement and voltage errors. Occasionally, it is desirable to mount a sensor directly to a printed circuit board. Calibration of such hard mounted sensors, however, is problematic.
In addition, in some types of electronic control systems, a plurality of sensors are coupled to one or more analog-to-digital (A/D) converters through multiplexors. To facilitate signal processing, each sensor is coupled to an amplifier to produce a signal compatible with the A/D input. Each amplifier channel, however, typically adds error to the respective sensor signals, and multiplexing sensors together through a plurality of amplifiers increases sensor to sensor bias or error because of several modes of error associated with each of the amplifiers.
In an exemplary embodiment, a measurement system for use in, for example, an appliance control system includes a multiplexor, a plurality of sensors coupled to the multiplexor, and a single amplifier circuit having an impedance path. The multiplexor is configured to selectively place one of the sensors into the impedance path of the amplifier. In different embodiments, the sensors are placed into an amplifier input impedance path, and an amplifier feedback impedance path. Transfer functions of the amplifier circuits are dependent upon a selected sensor response when placed into the circuit, thereby allowing a controller to make control decisions based upon a voltage output of the amplifier. Because a single amplifier channel is used to read all the sensors, measurement error attributable to a plurality of amplifier channels is reduced.
In another aspect, the measurement system includes an integrated circuit having at least a first channel, a second channel, and a hard mounted sensor directly mounted to the first channel. An adjacent channel calibration technique is used to calibrate the hard mounted sensor by coupling a removable sensor to the second channel and calibrating the second channel by measuring signals with the removable sensor and at least one known equivalent source, such as a tight tolerance resistor. Using the known and expected values, a sensor offset value is determined for the second channel, and the sensor offset is applied to both the first channel and the second channel as the respective sensors are read by the controller. A cost effective hard mounted sensor is therefore provided that may be calibrated before the circuit board is installed in the field.
Therefore a measurement system with reduced error provides increased measurement accuracy to meet increased performance objectives of, for example, major appliances including a plurality of thermistors for selection, execution, and feedback control of operating algorithms of an appliance.